Interest has recently increased in the development of implantable defibrillators that may be inserted entirely subcutaneously or sub-muscularly, having no leads or electrodes within the thoracic cavity. The elimination of transvenous or epicardial leads is believed likely to allow for implant of the devices by a wider range of physicians, in some cases at a lower cost than traditional implantable cardio defibrillators (ICDs). Such devices, are therefore believed to offer the opportunity for increased levels of use, particularly for prophylactic implant. US Application Publication Nos. 2002/0042634, 2002/00068958 and 2002/0035377 to Bardy et al., are exemplary of current thinking with regard to such subcutaneous ICDs. Additional subcutaneous ICDs are disclosed in US Application Publication No. 20020082658 by Heinrich et al. and PCT publication WO/04043919A2 by Olson. All of the above cited applications and publications are incorporated herein by reference in their entireties.
The above-referenced Bardy et al. applications disclose subcutaneously implanted ICD IPGs that are coupled with at least one cardioversion/defibrillation lead. In certain embodiments, the ICD IPG has a conventional configuration having a can electrode that functions as one cardioversion/defibrillation electrode and is implanted subcutaneously anterior or posterior to the heart. The cardioversion/defibrillation lead is tunnelled subcutaneously under the skin and around the thorax to locate the lead supported cardioversion/defibrillation electrode posterior or anterior to the heart, respectively. In certain embodiments, two cardioversion/defibrillation leads that are electrically connected together are tunnelled subcutaneously under the skin and around the thorax to locate the two cardioversion/defibrillation electrodes apart from one another and posterior or anterior to the heart, respectively. Electrical sensing of the cardiac electrical activity is accomplished across two sense electrodes displaced apart from one another on the IPG housing or on the lead. Cardioversion/defibrillation shocks are delivered across the thorax between the cardioversion/defibrillation electrodes on the ICD housing and the lead. It is also asserted that cardiac pacing pulses can be applied to the heart across the cardioversion/defibrillation electrodes on the ICD housing and the lead. In certain embodiments, the ICD housing is shaped in an elongated, thin, narrow shape to approximate and conform to the curvature of the thorax for cosmetic reasons and in some cases to fit between the ribs, e.g., between the fourth and fifth ribs. In some such embodiments, the ICD may have no associated subcutaneous lead and may have both cardioversion/defibrillation electrodes mounted to the ICD housing.
While the above-cited applications by Bardy et al generally propose that pacing be done using the large surface area cardioversion/defibrillation electrodes, the Olson publication proposes that pacing may be accomplished using two smaller electrodes. One of these electrodes may be located on each of two separate device housings that are coupled to one another by a subcutaneous lead. Alternatively, one of the pacing electrodes may be located on the subcutaneous lead.
Like transthoracic pacing, for example as disclosed in U.S. Pat. Nos. 4,349,030, and 5,018,522, subcutaneous pacing has the potential to cause discomfort to the patient, as well as phrenic nerve and/or muscular stimulation, including direct diaphragmatic stimulation. This drawback may limit the use of subcutaneous pacing therapies, including anti-tachycardia, anti-bradycardia or post-shock pacing, in some patients.